A cellular network consists of multiple base stations (BSs, or simply, “cells”) controlled by a common radio resource management (RRM) unit. The base stations transmit signals to multiple subscriber (mobile) stations (SS, or simply, “users”) over a predetermined frequency range and bandwidth. The transmission time is divided into equal frames, which may be sub-divided into shorter time intervals. Likewise, the bandwidth may be sub-divided into equal sub-channels. Taken together, this time-frequency division may be thought of as a periodic frame structure consisting of multiple slots, which occupy certain time and frequency resources. Each “BS, slot” pair may be allocated to some subscriber station, meaning that this base station will transmit a signal to this subscriber station in this slot.
In wireless communication systems, the same slot is usually reused among neighboring base stations. In downlink transmissions (from the base station to one or more subscriber stations), there may be a strong interference from one base station to subscriber stations served by its neighboring base stations (NBS).
In a given slot, the interference to subscriber stations is determined by the location of the subscriber station and by the signal power of all base stations (assumed to be constant over frames). This motivates the notion of a reuse pattern of a slot, being defined as a vector comprising the powers of all base stations (in this slot). If each base station transmits either full power or zero power, then the reuse pattern is equivalent to the set of base stations transmitting full power. The frame is separated into a few partitions, each partition including all slots with a specific reuse pattern.
Under non-fractional frequency reuse (FFR) technology, each base station transmits in a single frame partition, but leaves other partitions unused. The FFR technique is used to improve the system spectral efficiency (vs. a non-FFR system with many partitions) or cell edge coverage (vs. a non-FFR system with a single partition). Under FFR, every base station transmits in many (all) frame partitions, benefiting from the diversity of interference conditions at the users' locations.
Under non-FFR technology, the network selects for each subscriber station its serving base station (SBS). Under FFR, however, the network selects the partition of the serving slot, together with the SBS selection. In other words, the FFR network selects a “SBS, partition” pair for each subscriber station.
Typically, a non-FFR network selects the SBS with the best spectral efficiency (SE) at the location of the user. This “select the best” rule is optimal and consistent with the structure of the non-FFR network structure. That is, the rule results in a near even “load” of base stations, where “load” means the number of served users. As used herein, when a load of base stations is deemed “near even,” this means that the load of each base station is equal or approximately equal to the load of any other base station.
But the “select the best” rule is not applicable in an FFR network. Indeed, following this rule, the selected SBS would always be accompanied by the partition with the highest SBS power, and relatively lower power of the neighboring base stations. So, such “BS, partition” pairs would end up being overloaded, while other “BS, partition” pairs would have no load.
To address the load balancing problem, some selection rules are proposed that force the user to select a pattern that is not the best. These prior art solutions are sub-optimal and depend on many engineered parameters, in contrary to the simple “select the best” rule.
Thus, there is a need for an optimal resources management rule to be used in a fractional frequency reuse network.